cache: stop requeuing negative entries on the hot list
Turns out it does not improve hit ratio, but it does come with a cost
induces stemming from dirtying hit entries.
Sample result: hit counts of evicted entries after 2 buildworlds
before:
value ------------- Distribution ------------- count
-1 | 0
0 |@@@@@@@@@@@@@@@@@@@@@@@@@ 180865
1 |@@@@@@@ 49150
2 |@@@ 19067
4 |@ 9825
8 |@ 7340
16 |@ 5952
32 |@ 5243
64 |@ 4446
128 | 3556
256 | 3035
512 | 1705
1024 | 1078
2048 | 365
4096 | 95
8192 | 34
16384 | 26
32768 | 23
65536 | 8
131072 | 6
262144 | 0
after:
value ------------- Distribution ------------- count
-1 | 0
0 |@@@@@@@@@@@@@@@@@@@@@@@@@ 184004
1 |@@@@@@ 47577
2 |@@@ 19446
4 |@ 10093
8 |@ 7470
16 |@ 5544
32 |@ 5475
64 |@ 5011
128 | 3451
256 | 3002
512 | 1729
1024 | 1086
2048 | 363
4096 | 86
8192 | 26
16384 | 25
32768 | 24
65536 | 7
131072 | 5
262144 | 0
Sponsored by: The FreeBSD Foundation
This commit is contained in:
mjg
parent
a67ed57908
commit
306794b4a0
@ -92,10 +92,10 @@ SDT_PROBE_DEFINE1(vfs, namecache, purge_negative, done, "struct vnode *");
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SDT_PROBE_DEFINE1(vfs, namecache, purgevfs, done, "struct mount *");
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SDT_PROBE_DEFINE3(vfs, namecache, zap, done, "struct vnode *", "char *",
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"struct vnode *");
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SDT_PROBE_DEFINE3(vfs, namecache, zap_negative, done, "struct vnode *",
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"char *", "int");
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SDT_PROBE_DEFINE3(vfs, namecache, shrink_negative, done, "struct vnode *",
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"char *", "int");
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SDT_PROBE_DEFINE2(vfs, namecache, zap_negative, done, "struct vnode *",
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"char *");
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SDT_PROBE_DEFINE2(vfs, namecache, shrink_negative, done, "struct vnode *",
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"char *");
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/*
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* This structure describes the elements in the cache of recent
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@ -109,7 +109,6 @@ struct namecache {
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struct vnode *nc_dvp; /* vnode of parent of name */
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union {
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struct vnode *nu_vp; /* vnode the name refers to */
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u_int nu_neghits; /* negative entry hits */
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} n_un;
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u_char nc_flag; /* flag bits */
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u_char nc_nlen; /* length of name */
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@ -132,7 +131,6 @@ struct namecache_ts {
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};
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#define nc_vp n_un.nu_vp
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#define nc_neghits n_un.nu_neghits
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/*
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* Flags in namecache.nc_flag
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@ -220,9 +218,6 @@ SYSCTL_UINT(_vfs, OID_AUTO, ncsizefactor, CTLFLAG_RW, &ncsizefactor, 0,
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static u_int __read_mostly ncpurgeminvnodes;
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SYSCTL_UINT(_vfs, OID_AUTO, ncpurgeminvnodes, CTLFLAG_RW, &ncpurgeminvnodes, 0,
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"Number of vnodes below which purgevfs ignores the request");
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static u_int __read_mostly ncneghitsrequeue = 8;
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SYSCTL_UINT(_vfs, OID_AUTO, ncneghitsrequeue, CTLFLAG_RW, &ncneghitsrequeue, 0,
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"Number of hits to requeue a negative entry in the LRU list");
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static u_int __read_mostly ncsize; /* the size as computed on creation or resizing */
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struct nchstats nchstats; /* cache effectiveness statistics */
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@ -676,8 +671,6 @@ SYSCTL_PROC(_debug_hashstat, OID_AUTO, nchash, CTLTYPE_INT|CTLFLAG_RD|
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*
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* A variation of LRU scheme is used. New entries are hashed into one of
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* numneglists cold lists. Entries get promoted to the hot list on first hit.
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* Partial LRU for the hot list is maintained by requeueing them every
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* ncneghitsrequeue hits.
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*
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* The shrinker will demote hot list head and evict from the cold list in a
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* round-robin manner.
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@ -686,28 +679,12 @@ static void
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cache_negative_hit(struct namecache *ncp)
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{
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struct neglist *neglist;
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u_int hits;
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MPASS(ncp->nc_flag & NCF_NEGATIVE);
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hits = atomic_fetchadd_int(&ncp->nc_neghits, 1);
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if (ncp->nc_flag & NCF_HOTNEGATIVE) {
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if ((hits % ncneghitsrequeue) != 0)
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return;
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mtx_lock(&ncneg_hot.nl_lock);
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if (ncp->nc_flag & NCF_HOTNEGATIVE) {
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TAILQ_REMOVE(&ncneg_hot.nl_list, ncp, nc_dst);
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TAILQ_INSERT_TAIL(&ncneg_hot.nl_list, ncp, nc_dst);
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mtx_unlock(&ncneg_hot.nl_lock);
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return;
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}
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/*
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* The shrinker cleared the flag and removed the entry from
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* the hot list. Put it back.
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*/
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} else {
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mtx_lock(&ncneg_hot.nl_lock);
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}
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if (ncp->nc_flag & NCF_HOTNEGATIVE)
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return;
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neglist = NCP2NEGLIST(ncp);
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mtx_lock(&ncneg_hot.nl_lock);
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mtx_lock(&neglist->nl_lock);
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if (!(ncp->nc_flag & NCF_HOTNEGATIVE)) {
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numhotneg++;
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@ -851,8 +828,9 @@ cache_negative_zap_one(void)
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blp != NCP2BUCKETLOCK(ncp2) || !(ncp2->nc_flag & NCF_NEGATIVE)) {
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ncp = NULL;
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} else {
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SDT_PROBE3(vfs, namecache, shrink_negative, done, ncp->nc_dvp,
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ncp->nc_name, ncp->nc_neghits);
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SDT_PROBE2(vfs, namecache, shrink_negative, done, ncp->nc_dvp,
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ncp->nc_name);
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cache_zap_locked(ncp, true);
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counter_u64_add(numneg_evicted, 1);
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}
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@ -887,8 +865,8 @@ cache_zap_locked(struct namecache *ncp, bool neg_locked)
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if (ncp == ncp->nc_vp->v_cache_dd)
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ncp->nc_vp->v_cache_dd = NULL;
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} else {
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SDT_PROBE3(vfs, namecache, zap_negative, done, ncp->nc_dvp,
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ncp->nc_name, ncp->nc_neghits);
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SDT_PROBE2(vfs, namecache, zap_negative, done, ncp->nc_dvp,
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ncp->nc_name);
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cache_negative_remove(ncp, neg_locked);
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}
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if (ncp->nc_flag & NCF_ISDOTDOT) {
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